The several facilitating and inhibiting effects of preconditioning on the human, reflex startle-blink offer the possibility that different levels of processing may be distinguished and may elucidate the nature of changes occurring in early development. Three phasic control systems, distinguished in terms of eliciting-stimulus characteristics and by their effects on cardiac rate changes, are assumed to be important determinants of cognitive ability. A short-time constant system, sensitive to transient change in stimulation, is hypothesized to function as an interrupt related to pre-attentive processing. A long-time constant system, capable of temporal integration and sensitive to sustained intensity, is viewed as the phasic aspect of a classical activation, output-enhancing system, and an orienting-attentional system, sensitive to stimulus information, is viewed as enhancing input. The three systems have independent effects on reflex blinking in adults, with the transient system inhibiting, the activational facilitating, and the attentional facilitating or inhibiting depending on the direction of attention. The research will determine development of the preconditioning effects during infancy, whether or not the systems elicit differential patterns of pretarsal and orbital electromyographic activity in the early oligosynaptic and in the late, polysynaptic components of the reflex, whether effects vary as a function of sleep and sleep stage, and whether sleep stage effects interact with maturity.